The present invention relates to a method and apparatus for determining the state-of-charge of batteries. The invention is particularly applicable with respect to lithium batteries, such as Li/SO.sub.2 and Li/SOCL.sub.2 batteries, and is therefore described below with respect to this application, although it will be appreciated that the invention may also be advantageously used for other batteries as well.
It is frequently very important to determine the state-of-charge of a battery, i.e. the residual capacity (ampere-hours) still remaining. However, some batteries, particularly the lithium batteries such as Li/SO.sub.2 and Li/SOCL.sub.2 batteries, have very constant or flat discharge curves which give no information with respect to the battery state-of-charge. Insofar as we are aware, no practical, non-destructive technique has been devised for measuring the state-of-charge of lithium batteries or other batteries having constant discharge curves. In addition, in the case of Li/SO.sub.2 batteries, the open circuit voltage (OCV) is independent of the state of charge in the range of 60-100% capacity. For example, a fully charged 10 Ah Li/SO.sub.2 cell has the same OCV (2.993 0.003 V) as that of a 40% discharged cell. Thus, it is impossible to predict the state of charge from the OCV.
An object of the present invention is to provide a novel method, and also novel apparatus, for determining the state-of-charge of batteries, which method and apparatus are particularly useful with respect to lithium batteries or other batteries having very constant discharge curves.